Electroluminescent (EL) phosphors are incorporated into numerous thick-film, alternating-current, EL display devices. One particularly important class of EL phosphors is the copper-activated zinc sulfide phosphors which include the commercially important ZnS:Cu,Cl and ZnS:Cu,Mn,Cl phosphors. In the manufacturing of these ZnS-based phosphors reference is typically made to at least a two-step firing procedure involving some kind of intermediate comminution or chemical treatment step between the two firing steps. For example, U.S. Pat. No. 2,745,811 teaches a synthesis method involving double firing with a re-mixing between the firing steps and U.S. Pat. No. 2,957,830 teaches a synthesis method involving a first-step firing at high temperatures, followed by light crushing of the material, and then a lower temperature second-step firing. U.S. Pat. No. 3,076,767 teaches a synthesis method involving a first-step firing at high temperatures, followed by a hydrogen peroxide etching step, and then by a low-temperature second-step firing. U.S. Pat. No. 3,082,344 (1963) teaches a two-step firing process wherein the first-step firing is conducted in a sulphurizing atmosphere at a high temperature, followed by a second-step firing (baking) at a lower temperature in an oxygen or air atmosphere, and then a subsequent cooling step involving rapid quenching in an air atmosphere.
More recently, attention has been focused on intermediate steps involving the application of mechanical force to the material from the first-step firing. For example, U.S. Pat. No. 4,859,361 teaches a synthesis method involving a first-step firing at high temperatures followed by a low-intensity milling step designed to change the crystallographic structure of at least a portion of the material After the low-intensity milling, the material is subjected to a low-temperature, second- step firing to form the phosphor. In a similar manner, U.S. Pat. No. 5,643,496 teaches a synthesis method involving a first-step firing at high temperatures followed by a low-intensity milling (or mulling) step designed to convert some of the hexagonal ZnS to its cubic crystalline form and then by subsequent low-temperature second-step firing. Comparable methods are also disclosed in U.S. Pat. Nos. 5,702,643 and 5,711,898. In Japanese Patent Application 02-094287, the mechanical force is applied through isostatic pressing. In particular, the synthesis method involves a plurality of steps including a first-step firing followed by isostatic pressing of the intermediate material at between 200 to 2000 kg/cm2 and then by a low temperature annealing. In Japanese Patent Application 09-059616, an impact force is applied to the particles by causing accelerated particles to collide with one another, e.g., jet milling.
The application of an intermediate mechanical force coupled with a low temperature second-step firing is thought to contribute to the development of line-shaped or stria-like crystal imperfections, also referred to as crystal stacking faults, that are associated with the luminescent lines or striae observed with EL phosphors. These luminescent lines, luminescent striae, and/or comet-like luminescent sites are critical parts of the observations by Gillson and Darnell, Phys. Rev. 125, 149 (1962), and the electroluminescence models developed by A. G. Fischer. See, e.g., A. G. Fischer, J. Electrochem. Soc. 109 (11), 1043–1049 (1962); A. G. Fischer, J. Electrochem. Soc., 110 (7), 733–747 (1963); and A. G. Fischer, Luminescence of Inorganic Solids, edited by Paul Goldberg, Academic Press, Yew York, chapter 10, 541–601 (1966). Much of this prior art is reviewed in the Phosphor Handbook, edited by S. Shionoya and W. Yen, CRC Press, NY, Chapter 9 (section two), 601–612 (1999).